A differential white cell count (DWCC) is an important diagnostic tool which can be used to distinguish between various conditions which induce alterations in the total number and type of white blood cells produced by the human body. For example, a DWCC can be used to distinguish between bacterial or viral infections, in the differential diagnosis of an allergic reaction or to detect the presence of myoproliferative disorders or leukemia on board an International Space Station, ISS.
Microgravity exposure during space flight results in hemodynamic changes in crew members, which in turn impacts upon the production of white blood cells. No data is presently available to establish the "normal baseline" for white blood cell production in microgravity. Without first knowing the extent to which microgravity exposure impacts white blood cell production, or secondly the proper "microgravity baseline" for a normal healthy crew-member in space, it is quite possible that a bacterial or viral infection may be overlooked or misdiagnosed, or that a potentially much more serious problem, such as leukemia, may be attributed to a bacterial or viral infection in a particular crew-member.
At the present time it is impossible to perform DWCC while aboard an orbiting space craft. Whole blood smears have been produced in microgravity, but as yet it has remained impossible to perform DWCC without returning the blood smear to Earth. Due to the limited life span of such smears it is impossible to make a definitive statement with regard to the effect of microgravity exposure upon white blood cell profile based on such samples. Until real-time performance and analysis of a DWCC can be achieved aboard the space craft, critical crew health information remains unobtainable.
In a terrestrial setting, a differential white cell count is obtained by firstly preparing a blood smear on a glass slide, fixing the cells in the smear to the surface of the slide, staining the cells with a histochemical stain followed by washing the slide in a clean buffer solution prior to viewing under the microscope where a differential white blood cell count is made by morphological criteria. The protocol outlined above is a simple and universally used technique to perform a DWCC. However, this technique requires the use of liquid buffer solutions, including fixatives and dye solutions. While this technique is performed easily on Earth, the problems associated with liquid handling in microgravity make such a task practically impossible.
Past attempts at solving this problem have induded several cell stainers which were tested by NASA or its contractor personnel but have since proved unsuitable for use in microgravity. The first attempt was a slide stainer which flew aboard Sky Lab. This device proved very cumbersome, required large volumes of buffer solutions and had limited use due to precipitate formation in the staining solutions which blocked the intricate tubing arrangement required to apply the staining solutions to the blood smear. A second attempt was based upon an airtight chamber design which contained a blood smear slide, into which buffer solutions and/or staining solutions were introduced using a vacuum system. System operation relied upon a series of one-way and two-way valves in order to achieve an efficient vacuum into which the staining solutions were introduced by hypodermic syringe. The original technology used a hand-held squeeze bulb to create the vacuum which proved inadequate. A later version incorporated mechanical pumps to provide both vacuum production and syringe emptying. The hand-operated version of this technology, although shown to work on the ground and which passed initial testing aboard the KC-135 parabolic aircraft, did not fulfill its potential; thus this technology has been shelved as a viable solution to slide staining on-orbit, not least because of its requirement for substantial crew interaction.